It is known that in a spread spectrum receiver, a matched filter or convolver is used as a correlator to demodulate a spread signal and subsequently integrate the demodulated output for a predetermined time to reduce influences of multipath transmission.
FIG. 6(A) shows that correlation spikes a-1 and a-2 appear in a demodulated output as a result of multipath transmission.
When using the signal as an input and effecting integration in a period longer than the distance between the two correlation spikes, energies of two correlation spikes are added as shown in FIG. 6(B), and the demodulated output is increased.
For example, a prior art PDI (Post Detection Integration) circuit is proposed by Robert E. Kahn in a report entitled "Advances in Packet Radio Technology" of "Proceedings of the IEEE, Vol. 66 No. 11, November, 1978" .
FIG. 7 is a block diagram of a PDI circuit arrangement including a matched filter 21, one-bit delay circuit 22, integrating circuit 23, zero-threshold circuit 24 and multiplier 25.
This shows an approach to data demodulation in a DPSK (differential phase shift keying) receiver where a matched filter output Y(t) (FIG. 8A) and a signal Z(t) (FIG. 8B) delayed by one bit from Y(t) are applied to and multiplied in the multiplier 25, and the signal Y(t)Z(t)[=U(t)] (FIG. 8C) is integrated in the period from A to B, both being repeated to effect data demodulation based on 0 (zero) level.
The integrating circuit 23 performs integration in the period from A to B, subsequently initializes (dumps) the integrated value once, and similar operations are further effected. That is, integration and dumping are repeated.
In this case, if the timing of the signal U(t) and the timing of the integration period from A to B are synchronous, all energy of U(t) is integrated, and data demodulation is attained (see FIG. 8D).
However, this is not accomplished in an initial synchronizing process before the synchronization is established. That is, when the integrating period from (A' to B') and the dumping period are not synchronous with the signal U(t) as shown in FIG. 8E, a signal inputted in the dumping period is not integrated, and part of information is lost.